Interaction of Pure Marangoni Convection with a Propagating Reactive Interface under Microgravity

A reactive interface in the form of an autocatalytic reaction front propagating in a bulk phase can generate a dynamic contact line upon reaching the free surface when a surface tension gradient builds up due to the change in chemical composition. Experiments in microgravity evidence the existence of a self-organized autonomous and localized coupling of a pure Marangoni flow along the surface with the reaction in the bulk. This dynamics results from the advancement of the contact line at the surface that acts as a moving source of the reaction, leading to the reorientation of the front propagation. Microgravity conditions allow one to isolate the transition regime during which the surface propagation is enhanced, whereas diffusion remains the main mode of transport in the bulk with negligible convective mixing, a regime typically concealed on Earth because of buoyancy-driven convection.

Figure 1

(a) Side view of the 10-mm-wide cell at 317 s after initiation of the reaction. This corresponds to the end of the microgravity period. The dark region indicates the product solution containing triiodide behind the reaction front propagating from left to right. (b) Temporal evolution of the reaction front in the 10-mm-wide cell. The time delay between neighboring front profiles is 14.85 s. The red dashed lines are the reconstructed front profiles based on the geometric spreading of a RD front with the contact line advancing at a greater velocity on the free surface. Blue arrows indicate the displacement directions, along which the appropriate values for the velocity of propagation have been determined. Flow field in the center $y\text{−}z$ plane of the cell superimposed over the false colored concentration field (c) in the experiment and (d) in the simulation. The measured fluid velocity in the bulk was obtained via PIV.

Figure 2

Comparison of the maximum horizontal fluid motion velocity $u_{\max }$ as a function of the depth along the vertical center line of the 10-mm-wide cell. Here, the gas-liquid interface is located at $z=1.0\mathrm{cm}$. The depth-dependent maximum fluid flow velocity $u_{\max }$ (full circles) was determined via PTV for particles in the proximity of the surface. Also, shown with black squares are the results of the model calculation for $M=2$ rescaled to the appropriate dimensional coordinates with $l_c=0.125\mathrm{mm}$ and $t_c=2.8\mathrm{s}$ [1, 32]. $u_{\max }$ is defined as the horizontal velocity with maximum absolute value; negative values of $u_{\max }$ hence corresponding to horizontal velocities directed to the left of the system.